1. Field of the Invention
This invention relates to a novel coupler for forming a yellow image which is used in color photography.
2. Description of the Prior Art
It is well known that in color photography, a coupler-containing light-sensitive photographic material, for example, is exposed and then color developed with a developer containing an aromatic primary amine type developing agent as a main ingredient to form a dye image. Among the couplers used in said photography, the yellow coupler has an active methylene group which serves to form a yellow dye by coupling with an oxidation product of the aromatic primary amine type developing agent. In case the said active methylene group has not been substituted (i.e. in the case of an unsubstituted type yellow coupler), 4 molecules of silver halide is required in order to form one molecule of dye in the color development. Thus, the above-mentioned yellow coupler is called a 4-equivalent coupler.
On the other hand, it is well known that the same dye as in the case of unsubstituted type coupler can be formed also from a so-called substituted type yellow coupler, i.e. a coupler having an active methylene group, one of the hydrogen atoms of which has been substituted by such a substituent as a chlorine atom or the like halogen atom. In this case, the halogen atom or the like substituent is released in the course of color development reaction, and one molecule of dye can be formed from 2 molecules of developed silver halide. Such substituted type yellow coupler as mentioned above is called a 2-equivalent coupler. This 2-equivalent yellow coupler has, for example, such advantages over the 4-equivalent yellow coupler as mentioned below.
(1) The coupling rate can be more increased than in the case of the known 4-equivalent yellow coupler. PA1 (2) The amount of silver halide required for forming a dye may be one half the amount required in the case of the 4-equivalent coupler, so that the production cost of photographic material can be reduced. PA1 (3) The emulsion layer can be made thinner to improve the resulting color image in resolution and sharpness. PA1 (4) In the case of a multi-layered photographic material, the transmission of light to the under layers is enhanced to improve the photographic speed of the photographic material.
Thus, the substituted type coupler is extremely advantageous for use in photography. On the other hand, however, the conventional substituted type coupler has any of such disadvantages that it tends to form fog or the like color stains and tends to disturb the development of photographic material.
In contrast to the conventional substituted type coupler, the substituted type coupler according to the present invention is colorless, is high in reactivity and scarcely forms color stains. Moreover, the yellow dye formed by the aforesaid color development is excellent in fastness to light, humidity and heat, has no unnecessary absorptions in the long wavelength region, shows less and sharp absorptions in the green light region, and has a color tone quite favorable for color reproduction.